Demands from the microelectronics industry are driving for lower dielectric constant (low k) materials for use as insulating interlayers between conductive lines and components of integrated circuits (IC) and associated electronic devices. Conductive line dimensions are being reduced in these products in order to increase the speed and memory storage capability of microelectronic devices, e.g., computer chips.
Chemical Vapor Deposition (CVD) and spin-on methodologies have been used as primary methods of forming low dielectric films, with k=1.8-3.5, in IC production. These methodologies are very different from each other requiring different chemistries, precursors, and equipment. In general, in a CVD process vaporized organosilicate precursors are delivered to a reaction chamber through a showerhead. Through the application of an energy source (plasma, thermal, etc) to the reaction chamber, the precursor is activated generating a solid film on the substrate. The deposition typically is done at low pressure (<20 torr) and at high temperature (200-400° C.). On the other hand, a spin-on process generally utilizes a precursor, i.e., polymers, organometallic, inorganic, hybrids of inorganic and organic, dispersed or dissolved in a solvent. The solution is applied to a spinning wafer at ambient temperature and pressure, and then thermally treated to remove the solvent and drive the reaction toward completion. The required deposition equipment, chemistry, and physical deposition process is very different for these two techniques.
The major advantage of the CVD technique is that it is the incumbent technology and existing toolsets can be used for subsequent generations of films. The major disadvantage of CVD is that it is difficult to form low dielectric films, particularly of the porous type, e.g., an organosilicate glass (OSG) film (k<2.7), by CVD techniques due to the volatility requirements of the precursors and the deposition conditions of the process. The major advantage of a spin-on process is the quick processing time, the ability to use non-volatile precursors in the formulation, the resulting uniformity of the film across large substrates, and the preservation of the precursor structure. The biggest disadvantages of spin-on are the requirement of a separate toolset, the low efficiency of chemical usage, and the problems associated with disposal of chemical waste.
Liquid source misted chemical deposition technology (LSMCD) is a new route to high dielectric films, such as metal oxides, ferroelectrics, superconductors, and silicon dioxide. Liquid source misted deposition utilizes a nebulizer, venturi or other means to produce mist droplets of precursor solution that are extremely fine and controlled. These mists of precursor solution are flowed through a deposition chamber and deposited onto a substrate. The solvent is driven from the deposited mist and a film is formed.
Representative patents illustrating apparatus and methods for depositing films using the liquid misted technique and patents illustrating the preparation of low dielectric films are as follows:
U.S. Pat. No. 5,456,945 discloses a method for depositing thin films of complex compounds such as metal oxides, ferroelectrics, etc. via liquid mist technology. The patentees point out that vacuum deposition, e-beam, laser ablation, CVD and liquid applications employing spin-on techniques, dipping and spraying have been used for forming such films but each posed significant disadvantages. In the liquid misted process, a precursor liquid is formed including an element, e.g., a metal-alkoxide and alcohol solvent, converted to a mist and deposited onto a substrate, e.g., a silicon wafer. During deposition UV radiation or a DC bias is applied between two parallel plates. After deposition, the solvent is removed and a bake cycle effected. Exemplary high dielectric films are of the lead zirconium titanate and barium strontium titanate type.
U.S. Pat. No. 5,759,923 discloses the fabrication of thin films of silicon dioxide and silicon glass by a liquid mist technique. In the disclosed process a precursor liquid including a silicon or silicon glass forming component, e.g., an alkoxy silane such as silicon butoxide is converted into a mist, flowed into a deposition chamber, and then the mist deposited over the surface of the substrate. Upon heating, the solvent is removed and films are formed by the application of heat or UV radiation.
U.S. Pat. No. 6,116,184 and WO 97/44818 disclose an improvement to the U.S. Pat. No. 5,456,945 liquid mist process when electric fields are used to assist in the deposition of such mists. Mists incorporating metal-organic compounds such as alkoxides and carboxylates do not ionize well making them difficult to use them in conjunction with an electric field. To solve the problem of poling (sic) of the mist, the patentees create a mist in a venturi that also ionizes the mist particles. The mist is passed through a velocity reduction chamber and showerhead type inlet plate in order to provide a uniform flow of mist to the substrate. An electrical accelerator is employed to increase the energy of the mist particles in a controlled manner so as not to break the chemical bonds that lead to high quality films. Oxygen is added to the mist to facilitate applying a charge to the mist.
US 2003/0118947 discloses the liquid source misted deposition of high dielectric films on select portions of a substrate. Select deposition is in contrast to U.S. Pat. No. 6,116,184 where the entire surface is coated. A polarized precursor mist migrates to an exposed or unexposed film portion of the substrate.
The following references describe several approaches to organosilicate glass (OSG) mixtures leading to low dielectric constant films, and methods of film deposition thereof.
U.S. Pat. Nos. 6,592,980 and 6,365,266 discloses ceramic films, particularly mesoporous films of increased porosity and low halide content, having low dielectric constants. A spinning solution containing a ceramic precursor, e.g., tetramethoxysilane, titanium isopropoxide, aluminum sec-butoxide and the like in a solvent such as a lower alkanol, an acid catalyst and a surfactant is applied to a substrate. Solvent is removed in the spinning process and, on calcination, the surfactant is removed to form a porous silica network.
U.S. Pat. No. 6,576,568 and 2004/0087184 disclose a process for depositing low dielectric porous silicon oxide-based films using a sol-gel approach. A stock precursor solution is formed which includes a soluble silicon oxide source, water, solvent, a purified nonionic surfactant, an ionic additive or amine additive and acid catalyst. The precursor solution is coated onto the substrate via spinning and after centrifugal draining the coated substrate is treated to form a hardened film.
U.S. Pat. No. 6,455,443 discloses a process for producing low dielectric films onto a substrate by applying a silane coupling agent containing a polymerizable group onto a surface of a substrate, heating the substrate to provide a surface containing Si—O bonds, rinsing the heated substrate with a solvent for removing residual silane-coupling agent and applying dielectric material to the rinsed surface containing Si—O bonds. Examples of low dielectric materials include polyarylene ethers, Si-containing polymers, e.g., organosilsesquioxanes.
U.S. Pat. No. 6,583,071 discloses a process for forming extremely low dielectric films (dielectric constants of 3 and below) via the coating of a substrate with a precursor solution comprised of a silicon oxide, solvent, surfactant, and acid catalyst using an ultrasonic spray nozzle. The components of the precursor solution are combined immediately prior to coating and the ultrasonic spray coating apparatus is scanned across the surface of a spinning substrate. A Sonotek Ultrasonic Atomizing Nozzle 8700-120MS is used to generate precursor solution droplets (median drop diameter ˜13-18 microns). The solvent and surfactant are removed on application of heat and a solid, porous low dielectric film is formed.